Sealing arrangement for high voltage bushings



March 29, 1960 D. L. JOHNSTON ETA!- 2,930,332

SEALING ARRANGEMENT FOR HIGH VOLTAGE BUSHINGS Filed May 23, 195

FrQ77c/Is C. 20 15M 22 United States Patent SEALING ARRANGEMENT FOR HIGH VOLTAGE BUSHINGS Donald L. Johnston and Francis C. Roy,

assignors to General Electric of New York Pittsfield, Mass, Company, a corporation This invention relates to high voltage electrical insulating bushings, and more in particular to an improved arrangement for providing a seal between the central conductor and bottom washer of a high voltage electrical insulating bushing.

High voltage electrical insulating bushings are generally comprised of a central conductor extending axially therethrough, at least the central portion of the central conductor being closely surrounded by a core of insulating material such as a plurality of layers of paper. The external surface of such bushings comprise a terminal assembly disposed on one end of the central conductor, a centrally disposed metallic mounting flange coaxial with the central conductor, a ceramic insulating shell extending between the upper terminal assembly and the mounting flange, a bottom washer disposed on the other end of the central conductor, and another ceramic insulating shell extending between the mounting flange and the bottom washer. The insulating shells and mounting flange define a dielectric fluid filled chamber surrounding the insulating core and central portion of the central conductor.

The present invention is specifically directed to the means for afiixing the bottom washer to the end of the central conductor. In the past, it has been common to thread the bottom washer on the end of the central conductor, the bottom washer being solder sealed to the central conductor. In order that mechanical stresses would not develop in the solder seals, the central conductor was threaded down solidly against a machined ledge in the bottom washer. It has previously been the practice in the fabrication of such bushings to completely assemble the structural elements of the bushing and solder the bottom washer prior to impregnation of the insulating core with an insulating fluid. In other words, the cores were dry when assembled, and any treatment of the core to impregnate it with a dielectric fluid such as insulating oil to eliminate voids in the insulating core was accomplished after the bushing was assembled. With such treatment occurring subsequent to the assembly, the operation of soldering the bottom washer to the central conductor was not diflicult, and provided a satisfactory seal.

It has now been found desirable, however, to impregnate the core with dielectric fluid prior to assembly, for example, by a thermal vacuum process, in order to reduce the dange of voids in the core and minimize the equipment necessary to treat the bushing.

The previously employed bottom washer seal has not been found satisfactory in application on bushings having pretreated cores, however, since soldering in the presence of the oil soaked core is dangerous from the standpoint of a tire hazard, and since the central conductor assemsly is coated with oil it is messy and diflicult to handle, and also difiicult to clean sufficiently to make a good solder seal.

It is therefore an object of this invention to provide an improved arrangement for sealing high voltage electrical insulating bushings.

It is a further object to provide an improved means for sealing the bottom washer to the central conductor of a high voltage electrical insulating bushing, the seal being characterized by not requiring a soldering operation that may present a fire hazard when the bushing core is pretreated with insulating oil.

A still further object of this invention is to provide a mechanically strong seal between the bottom washer and central conductor of a high voltage electrical insulating bushing, the seal not requiring the application of solder thereto.

Briefly stated, in accordance with one aspect of the invention, we provide a high voltage electrical insulating bushing comprising a central conductor. The central conductor has a first threaded portion adjacent one end thereof, a second threaded portion adjacent the first threaded portion, and a ledge extending between the threaded portions. The second threaded portion has a larger diameter than the first threaded portion. An insulating shell radially surrounds at least the portion of the conductor adjacent the threaded end. Washer means are provided having a central aperture, the aperture having one end portion threaded on the second threaded portion of the central conductor. The aperture also has an intermediate reduced diameter portion axially contacting the ledge between the threaded portions of the central conductor. The other end portion of the aperture of the washer is enlarged with respect to the diameter of the first threaded portion of the conductor, the enlarged end portion of the washer surrounding the first threaded portion of the central conductor. Annular gasket means are provided in the enlarged diameter portion of the washer, and nut means threaded on the first threaded portion of the central conductor holds the gasket means in sealing relationship between the first threaded portion of the central conductor and the reduced diameter portion of the washer.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which we regard as our invention, it is believed that the invention will be better understood from the following description taken in connection with the accompanying drawing. y

In the drawing:

Fig. l is a partially cross-sectional view of a high voltage electrical insulating bushing embodying the seal arrangement of the invention, and I Fig. 2 is an enlarged partially cross-sectional view of the bottom washer sealing arrangement of the bushing of Fig. 1. Referring now to the drawing, and more in particular to .Fig. 1, therein is illustrated a high voltage electrical insulating bushing comprising a central conductor 10 extending axially through the bushing. The conductor 10 may be hollow if desired to permit a conducting cable to extend therethrough. The central portion of the central conductor is closely surrounded by an insulating core 11, the core having the greatest radial dimension centrally of the bushing, and being tapered at its axial extremities as in the conventional practice. In the type of bushing illustrated in Pig. 1, it is preferred that the core be impregnated with a dielectric fluid and may thus be formed of a plurality of layers of paper tightly wound about a central conductor, the rolls of paper being interleaved with a plurality of conducting equalizing members. It is to be understood, however, that the present invention is not limited to any particular construction of insulating core.

The external surface of the bushing of Fig. l is comprised of a terminal assembly 12 on the upper end thereof, a centrally located metallic mounting flange 13, and a hollow ceramic insulating shell 14 extending between the terminal assembly 12 and the mounting flange 13. The mounting flange may have an extension 15 toward the bottom end of the bushing, the extension 15 serving as a ground shield. A bottom washer assembly 16 is disposed on the lower end of the central conductorlO, and another hollow ceramic insulating shell 17 extends between the mounting flange 13 and the bottom washer assembly. The terminal assembly 12, hollow ceramic insulating shells 14 and 17, mounting flange 13, and bottom washer assembly 16 definea dielectric fluid filled chamber 18 radially surrounding the central conductor and insulating core 11.

Referring now to Fig. 2, the lower end 19 of the central conductor is provided with a first set of external threads 20 adjacent the end, 19, and a second set of external threads 21 separated from the first set of threads 20 by a radially extending ledge 22. The second set of threads 21 has a' larger diameter than the first set of threads 20. V

A bottom washer 23 has an aperture with one end threaded to engage the threads on the second set of threads 21 of the central conductor. The aperture in the washer has a central reduced diameter or step portion 24 extending radially inwardly from the threaded end portion and axially (with respect to the bushing) engaging the ledge 22 of the central conductor. The other end 25 of the aperture in the washer 23 toward the end 19 of the centrai conductor is recessed and has a larger diameter than the step portion 24. A nut 26 engaging the first threads 26 of the central conductor is threaded tightly against the step portion of the bottom washer 23 within the recess portion 25 of the washer, the nut 26 holding an annular gasket 27 in sealing relationship between the step portion 24 of the bottom washer 23 and the first threaded portion of the central conductor.

A set screw 30 may be provided in the bottom washer to prevent rotation of the bottom washer on the threads of the central conductor. A plugged aperture 31 may also be provided extending generally axially through the bottom washer to permit draining of the dielectric fluid from the bushing.

The bottom washer 23 extends radially from the central conductor to axially engage the end of ceramic shell 17, the usual gaskets 32 providing a fluid tight seal between the bottom washer and ceramic insulating shell 17. The radially outer extremities of the bottom Washer may be externally threaded in order to facilitate connecting the lower end of the bushing to a conductor of an electrical apparatus on which the bushing is mounted.

When the bottom washer is assembled on the central conductor, the step portion of the washer is tightly held against the ledge between the threaded portions of the central conductor. This axial engagement between the step portion of the washer and ledge of the conductor provides a mechanically strong joint, prevents wobbling of the bottom washer, and removes mechanical stresses from the seal provided by the gasket 27. Since no soldering is required in order to seal the bottom washer, the bottom washer may safely and easily be assembled on the central conductor even though the core of the bushing has been pretreated with insulating oil.

It will be understood, of course, that, while the form of the invention herein shown and described constitutes the preferred embodiment of the invention, it is not intended herein to illustrate all of the possible equivalent forms or ramifications thereof. It will also be understood that the words used are words of description rather than of limitation, and the various changes may be made Without departing from the spirit or scope of the invention herein disclosed, and it is aimed in the appended claims to cover all such changes as fall within the true spirit and scope of the invention. V

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In a high voltage electrical insulating bushing having a central conductor extending axially therethrough and at least a portion of said central conductor extending from one end thereof being radially surrounded by an insulating shell, and means providing a fluid tight seal between said central conductor and insulating shell comprising first and second threaded portions on said central conductor, said first threaded portion being adjacent said end and separated from said second threaded portion by a radially extending ledge, the diameter of said second portion being greater than said first portion, a washer threaded on said second portion and extending into axial contacting relationship with said shell and having a radially inwardly extending step in axial contacting relationship with said ledge, and annular internal recess in the end of said washer toward said end of said conductor, gasket means, and nut means on said first threaded portion holding said gasket means in said recess and in sealing relationship between said washer and said first threaded portion of said conductor.

2. A high voltage electrical insulating bushing comprising a central conductor extending axially therethrough and at least one end thereof being radially surrounded by an insulating shell, and means providing a fluid tight seal between said end of said conductor and said shell com.- prising a first threaded portion adjacent said end of said conductor, a second threaded portion on said conductor adjacent said first threaded portion and having a larger diameter than said first threaded portion, an annular ledge between said threaded portions, a washer threaded on said second threaded portion having an internal step portion radially engaging said ledge and an enlarged diameter portion surrounding said first threaded portion, the radially outer portion of said washer axially engaging said shell, a gasket on said first threaded portion, and a nut threaded on said first threaded portion of said conductor within said enlarged diameter portion of said washer holding said gasket in sealing relationship between said washer and conductor.

References Cited in the file of this patent UNITED STATES PATENTS 

